In high pressure retaining systems, such as subsea production and workover equipment, situations may occur when the system pressure needs to be reduced to a required level, e.g., through choking. Rapid depressurization may also be encountered in some units, e.g., quick disconnect pressure control packages installed between a Christmas Tree (XT) or Wellhead (WH) and a riser string, when an emergency shutdown function is activated.
When gas flow is decreased by a choke valve, or it is released through a blow-down valve with a high pressure differential between the upstream and downstream sides of the plugs, the Joule-Thomson effect takes place, manifesting itself in a sharp temperature drop. Although the cooling time may be rather short (typically less than an hour), it is very detrimental to elastomeric seals, which are vulnerable to large temperature variations. In fact, any shock or undesirable movement could crack or rupture an elastomeric composite seal when it is cooled down to a brittle state.
Another point to consider here is the affect of regions with harsh weather conditions, in particular those of high alternating diurnal temperatures in mid seasons. In some areas of the Arctic the maximum daily temperature difference may reach 30-40° C. In the glassy or brittle state rubber seals may not necessarily be damaged, but, since the material may stiffen too much, the seal may lack sealability at low temperatures and, as a consequence, fail to separate the contained medium from the outer environment. The consequences of spillage of hydrocarbons or aggressive chemicals may undeniably be harmful either for people or flora and fauna, which are much more sensitive in cold areas. Hence, it is of utmost importance to maintain the temperature of elastomeric seals above their material glass transition temperature.
Elastomers with very low glass transition points, i.e. elastomers which tolerate low temperatures without becoming brittle, may be utilized in some instances. However, since in general the chemical resistance of such elastomers is inhibited, they are normally not suitable in high temperature wells or those producing extremely aggressive fluids, e.g., those containing a significant percentage of H2S in the gas phase. On the other hand, elastomeric composites with excellent high temperature stability exist which exhibit inertness to most of the production media as well as the chemicals utilized in the offshore industry, but their low temperature behavior is quite inferior.
The present invention aims to solve or alleviate the material selection challenge for sealing elements with high material glass transition points that might be temporarily subjected to the influence of low temperatures.